Liquid dispensers have been used in analyzers for the detection of the concentration of liquid analytes using as analysis means, test elements that contain within themselves the necessary reagents to permit such detection. Examples of such analyzers are described in U.S. Pat. Nos. 4,287,155, issued Sept. 1, 1981, and 4,340,390, issued July 20, 1982. Examples of such test elements appear in U.S. Pat. Nos. 3,992,158, issued Nov. 16, 1976., 4,053,381, issued Oct. 11, 1977; and 4,258,001, issued Mar. 24, 1981. The conventional method for dispensing liquid onto such test elements using such analyzers has been to aspirate test liquid from a relatively large container, into a dispensing container. The dispensing container is then moved to a position immediately above such a test element, and a fraction (e.g., 10 .mu.l) of the aspirated liquid is dispensed. The dispensing container is fluidly connected, in such analyzers, to a pressurizing means that generates both the operative partial vacuum needed to aspirate the needed amount of liquid into the container, and the partial pressure operative to dispense that aspirated liquid, in fractional amounts, onto a plurality of test elements. A pressure transducer is also conventionally included to ascertain the pressure within the container, so as to detect the occurrence of the desired dispensing event versus a failure to dispense. A microprocessor generally is used to control the apparatus in response to the conditions sensed.
Such conventional analyzers include a motor for raising and lowering the dispensing container, removably mounted on a probe, relative to the large container that supplies the test liquid. Such motors usually are preset to move the dispensing container a fixed distance into such large containers. This has functioned well when the level of the liquid within such large containers has been generally constant, and therefore predictable. However, usually the level is not constant. That is, although the large containers usually have a prescribed protocol that governs their filling, in the case where the dispensing apparatus is used for clinical analysis of body fluids, operators find it more convenient to overfill. Even the overfill is not necessarily constant. Because of the lack of predictability, the motor is preset to accommodate the lowest possible liquid level as the "nominal" liquid level. Unfortunately, this means that the exterior of the dispensing container becomes excessively wetted with the test liquid in those containers having more, and especially those with much more, than the minimum volume providing such lowest level. It has been found that such excessive wetting tends to encourage perfusion during subsequent dispensing. As used herein, "perfusion" means movement of the liquid being dispensed, up the exterior surface of the dispensing container, rather than down onto the test element. As is readily apparent, such perfusion prevents some or all of the desired test liquid from reaching the test element.
What then has been needed is a way of detecting when the dispensing container has penetrated the air-liquid interface within the large container. Although electrical contact of an electrically conductive dispensing container and the test liquid has been used in prior devices, such a technique requires dispensing containers made of especially conductive materials, which therefore become a permanent part of the device. In contrast, the dispensing containers disclosed in the aforesaid analyzer patents have been disposable after each test sample has been dispensed onto one or more test elements. Disposability is practically essential to prevent one test sample from contaminating another.
U.S. Pat. No. 3,894,438 discloses yet another method of detecting the penetration of the air liquid interface. In that patent, the sampling probe is provided with a sensing probe that is separate from but connected to the sampling probe so that the sensing probe enters the liquid phase after the sampling probe. A separate gas source is provided to the sensing probe, to cause an air stream to issue from the sensing probe. When the sensing probe reaches the air-liquid interface, the resistance to the outflowing air changes, and this change in pressure generates a signal that is indicative of the penetration having occurred.
The approach described in the '438 patent does permit the use of disposable dispensing containers. However, one drawback of such an approach is that it requires a second probe besides the sampling probe. Furthermore, a separate gas supply is also needed.
Thus, prior to this invention there has been a need for a simple mechanism for detecting the location of the air-liquid interface in sample supply containers having varying levels of liquids, that permits the use of disposable dispensing containers.